The present invention relates to a magnetic recording medium installed in various magnetic recording devices including an external memory of a computer, and to a manufacturing method therefor.
Non-magnetic substrates of magnetic recording media for magnetic recording devices conventionally employ an aluminum alloy substrate with electroless NiP plating or a glass substrate. So-called plastic substrates using plastic resin have been proposed for use as non-magnetic substrates of magnetic recording medium, as disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. H10-146843, for example. The plastic substrates, which can be manufactured economically and in a great mass by injection molding, are very promising for non-magnetic substrates of magnetic recording media that require further cost reduction.
When the plastic substrate is utilized as a non-magnetic substrate of a magnetic recording medium, since the temperature which the plastic substrate can withstand is low, at most below 200xc2x0 C., it is not acceptable to heat the plastic substrate to several hundred degrees Centigrade prior to depositing functional layers on the substrate. This temperature limitation is quite different from conventional substrates of aluminum alloy or glass. Accordingly, methods have been proposed to attain desired magnetic properties and electromagnetic conversion characteristics even with low temperature deposition. One example of such a method, as disclosed in Japanese Patent No. 2763165, employs a so-called granular magnetic layer. The granular magnetic layer is composed of a structure in which a grain of magnetic substance is surrounded by non-magnetic and non-metallic substances such as oxides or nitrides. In another example, disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. H11-154320, elevated argon gas pressure is employed in the process of depositing a granular magnetic layer by sputtering.
A magnetic recording device using a flying magnetic head must maintain a narrow gap between a magnetic recording medium and the head as small as a few tens of nanometers. Therefore, durability of the device is greatly affected by the friction and wear characteristic of the head-medium interface. To improve the friction and wear characteristic with a head, the surface of the medium is generally coated with liquid lubricant having a molecular weight of several thousands. It is known that the liquid lubricant on the medium surface can be dissolved as a result of cobalt atoms occasionally precipitated from the magnetic layer. Loss of the liquid lubricant results in significant deterioration of durability of the medium. A protective layer is conventionally deposited between the magnetic layer and the liquid lubricant for suppressing the precipitation of cobalt atoms. Control of thickness and quality of the protective layer and control of surface roughness of the medium are essential.
An object of the present invention is to provide a magnetic recording medium utilizing a plastic substrate, the medium exhibiting excellent magnetic property and an electromagnetic transformation characteristic, and at the same time, superior durability, and to provide a method for manufacturing such a recording medium.
Studies by inventors of the present invention revealed that the cobalt atoms contained in the magnetic layer of a magnetic recording medium readily precipitate on the medium surface when the medium is manufactured by the same method as is conventionally used with an aluminum alloy substrate or a glass substrate. The quantity of precipitated cobalt atoms is particularly large when the granular magnetic layer is employed as a magnetic layer, or when argon gas of elevated pressure is used in the sputtering process for attaining favorable magnetic property and electromagnetic conversion characteristics. The cobalt atoms, precipitated on the medium surface, decompose the molecules of the liquid lubricant on the medium surface, and thus deteriorate the resistance of the medium to friction and wear.
The inventors of the present invention made numerous studies for solving the above-described problems and have found that the amount of cobalt dissolved out and precipitated onto the medium surface strongly correlates with the material and the thickness of the under-layer of a magnetic recording medium.
More specifically, a magnetic recording medium of the present invention based on the above-described finding comprises a non-magnetic substrate made of plastic resin such as polycarbonate or polyolefin, a non-magnetic under-layer, a magnetic layer including cobalt, a protective film, and a liquid lubricant layer sequentially laminated on the substrate. The non-magnetic under-layer is composed of a chromium alloy that has a crystal structure of a body centered cubic lattice, and contains at least one element selected from the group consisting of Zr, Nb, Mo, Ru and Pd in a total amount of 15 at % or more.
In another aspect of the embodiments of the present invention, the non-magnetic under-layer is composed of a chromium alloy that has a crystal structure of a body centered cubic lattice, and contains at least one element selected from the group consisting of Hf, Ta, W, Re, Pt and Au in a total amount of 10 at % or more.
In a magnetic recording medium of the invention, an amount of cobalt dissolved out and precipitated onto the medium surface (also called simply xe2x80x98dissolved-out cobaltxe2x80x99) is 15 xcexcg/m2 or less. After exposing the medium to a high temperature and high humidity environment of 85xc2x0 C. and 80% relative humidity for 96 hours, the amount of dissolved-out cobalt is 20 xcexcg/m2 or less.
Thickness of the non-magnetic under-layer of the medium is preferably from 5 nm to 15 nm.
The inventors have further found that controlling argon gas pressure in at least one of the processes for depositing the magnetic layer and depositing the under-layer is effective to prevent the cobalt atoms from dissolving-out and precipitation onto the medium surface.
More specifically, a method for manufacturing a magnetic recording medium according to the present invention comprises a step for forming a non-magnetic under-layer on a non-magnetic substrate of plastic resin, a step for forming a magnetic layer on the under-layer, a step for forming a protective film on the magnetic layer, and a step for forming a liquid lubricant layer on the protective film. In the method, the non-magnetic under-layer is composed of a chromium alloy having a crystal structure of a body centered cubic lattice. The chromium alloy contains at least one element selected from the group consisting of Zr, Nb, Mo, Ru and Pd in a total amount of 15 at % or more, or contains at least one element selected from the group consisting of Hf, Ta, W, Re, Pt and Au in a total amount of 10 at % or more.
The step for forming an under-layer is preferably performed by sputtering under gas pressure of 30 mTorr or less. The step for forming a magnetic layer is preferably performed by sputtering under gas pressure of 15 mTorr or less. Most preferably, both of the gas pressure conditions are employed, that is, the step for forming an under-layer is performed by sputtering under gas pressure of 30 mTorr or less and the step for forming a magnetic layer is performed by sputtering under gas pressure of 15 mTorr or less.
Aspect of Embodiments of the Invention
The present invention will now be described in detail in the following with reference to a drawing.
FIG. 1 is a schematic cross-sectional view of a magnetic recording medium of the invention. Referring to FIG. 1, a magnetic recording medium of the invention comprises a plastic non-magnetic substrate 1, a non-magnetic under-layer 2, a magnetic layer 3, a protective film 4, and a liquid lubricant layer 5 sequentially formed on the substrate 1 in this order.
Meanwhile, the phrase xe2x80x9cquantity of cobalt dissolved out and precipitated onto the medium surfacexe2x80x9d is a value obtained by measuring the quantity of cobalt extracted by oscillating a sample of a magnetic recording medium in 50 ml of pure water for 3 min by means of ICP (inductively coupled plasma) emission spectroscopy. The measured value is represented by a mass of the cobalt per unit area of the medium surface in the unit xcexcg/m2.
A plastic substrate is employed as a non-magnetic substrate in the invention. The plastic substrate may be produced by injection molding polycarbonate resin or polyolefin resin, for example.
Initially, a non-magnetic under-layer 2 is formed on the plastic substrate 1. The material used for the under-layer is an alloy containing chromium as a major component and having a body centered cubic crystal structure in order to control the crystal structure of the magnetic layer 3. This chromium alloy contains at least one 4d transition metal element selected from a group consisting of Zr, Nb, Mo, Ru and Pd in a total amount of 15 at % or more. Alternatively, the chromium alloy contains at least one 5d transition metal element selected from the group consisting of Hf, Ta, W, Re, Pt and Au in a total amount of 10 at % or more.
The maximum quantity of the additives that retain the chromium alloy within a body centered cubic structure depends on the element to be added, which imposes a restriction on the maximum quantity of each additive element.
When the protective film 4 is thin or the film does not thoroughly cover the magnetic layer 5 due to large roughness of the magnetic layer, the cobalt atom in the magnetic layer generally tend to dissolve-out and precipitate onto the surface of the magnetic layer from the portion of the magnetic layer without the protective film or from inside of the magnetic layer by ordinal diffusion process through the thin protective film. However, even when the protective film appropriately covers the magnetic layer, according to the inventors"" findings, the cobalt atom in the magnetic layer may dissolve-out and precipitate onto the surface of the recording medium due to electrochemical reaction in the magnetic layer and at the boundary between the magnetic layer and the under-layer. Namely, the electrochemical reaction generates OH ions and Co ions, the former is originated from water molecules, and/or hydrogen atoms and oxygen atoms which are included during deposition of the layers or invaded into the layers by diffusion through the protective film after the deposition of the layers. The ionized cobalt atoms easily diffuse to the medium surface with the aid of OH ions through formation of a hydroxide with cobalt, resulting in enhanced amount of dissolved-out cobalt. Moreover, the electrochemical reaction, which makes the cobalt atom in the alloy lattice to dissolve-out, is strongly affected by the structure, particularly compactness and density not only of the magnetic layer, but also of the under-layer. That is, the under-layer with high compactness and density can suppress the electrochemical reaction and decreases quantity of dissolved-out cobalt. The addition of at least one transition metal elements as described earlier has been found to provide favorable structure of the layer for suppressing the electrochemical reaction and resulting to lowered quantity of dissolved-out cobalt, to obtain a magnetic recording medium exhibiting high durability using a plastic substrate.
The non-magnetic under-layer is advantageously deposited by sputtering process using argon gas under gas pressure of 30 mTorr or less. By employing such a process, compactness of the under-layer enhances and dissolving-out of cobalt atoms onto the medium surface is further suppressed.
Thickness of the under-layer of the invention is preferably in the range from 5 nm to 15 nm. If the thickness is less than 5 nm, a magnetic characteristic abruptly degrades with decrease of the thickness, while if thickness is more than 15 nm, an amount of the precipitating cobalt excessively increases.
On the under-layer 2, a magnetic layer 3 is formed. The magnetic layer is preferably a granular magnetic layer for attaining favorable magnetic property and an electromagnetic transformation characteristic because the granular magnetic layer provides a magnetic layer of excellent magnetic property even with low temperature deposition. However, the magnetic layer may also be composed of a Coxe2x80x94Cr alloy conventionally used together with an aluminum alloy substrate or a glass substrate.
The magnetic layer 3 is advantageously deposited by sputtering process using argon gas under gas pressure of 15 mTorr or less on the under-layer 2. By employing such a gas pressure condition, compactness of the under-layer enhances and precipitation of cobalt atoms onto the medium surface is further suppressed.
On the magnetic layer 3, a protective film 4 is formed. The protective film is a thin film including carbon as a principal component although not limited to this material. The protective layer is deposited by a commonly used sputtering process, for example.
On the protective film 4, a liquid lubricant layer 5 is formed. The liquid lubricant layer is formed of perfluoropolyether lubricant although not limited to this material.
The thus obtained magnetic recording medium of the invention exhibits excellent magnetic property and an electromagnetic transformation characteristic. The quantity of cobalt dissolved out and precipitated onto the surface of the magnetic recording medium is 15 xcexcg/m2 or less. After exposing the medium to a high temperature and humidity environment of 85xc2x0 C. and 80% relative humidity for 96 hours, the amount of dissolved-out cobalt is 20 xcexcg/m2 or less. Thus, the magnetic recording medium of the invention performs satisfactorily in long-term reliability.
The sputtering process for depositing the under-layer is conducted under an argon gas pressure of 30 mTorr or less. The sputtering process for depositing the magnetic layer is conducted under an argon gas pressure of 15 mTorr or less, as described above. Although only one of the two gas pressure conditions is effective, employing both of the two conditions brings about the most favorable result.
The magnetic recording medium of the invention may further include an intermediate layer between the magnetic layer 3 and the under-layer 2. The intermediate layer is a so-called seed-layer between the substrate and the under-layer, if required.
There is no particular restriction for material or thickness of the layers in the medium except for the under-layer, and commonly used material and thickness may be appropriately selected.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.